Purpose: To examine whether rest intervals of different durations (“High Rest, Low Frequency” v. “Low Rest, High Frequency”), and load at different force exertion levels with different repetition frequencies (“High Load, Low Repetition” v. “Low Load, High Repetition”) have an impact on muscle micro-trauma, as such micro-trauma over repetitive loading and long-term muscle overuse can lead to musculoskeletal disorders (MSDs).
Method: Twenty-four adult males (mean age: 24.1 years; 3.6 SD) were randomly assigned to one of 4 bicep muscle eccentric exercise treatment groups (n = 6; “High Load, Low Repetition; High Rest, Low Frequency”, “High Load, Low Repetition; Low Rest, High Frequency”, “Low Load, High Repetition; High Rest, Low Frequency”, or “Low Load, High Repetition; Low Rest, High Frequency”) with non-dominant arm to induce muscle micro-trauma reactions in serum. Subjects in all treatment groups had equivalent total work volume, total rest duration and total work duration for comparison of muscle micro-trauma between and within the treatment groups. Muscle micro-trauma biomarker serum Creatine Kinase (CK) level was measured pre-exercise (Day 0) and post-exercise on Days 1, 2, 4, and 8. ANOVA with repeated measures was used to examine significance of rest and load-repetition combination over pre and post experiment days, as well as possible interactions.
Result: CK levels fluctuated significantly across different “Day” (P = 0.0115). Interaction was disordinal and significant between “Day” and “Rest” (P = 0.0000), and “Load” and “Rest” (P = 0.0322). Under “High Load, Low Repetition” condition, CK levels on Day 4 were significantly higher than on Days 0 and 2; CK levels on Day 8 were significantly higher than Day 0. CK level peaked on Day 4. Under “Low Rest” Condition, CK level on Days 4 and 8 are significantly higher than Day 0.
Conclusion: Shorter but more frequent rest intervals led to more extreme muscle micro-traumatic responses than the longer but less frequent ones, especially under “High Load, Low Repetition” condition when non-dominant bicep brachii was subscribed with eccentric exercise regimen.
Relevance: to industry The exploration of how rest scheduling affects progression of microtrauma from a biomechanical and molecular level in this study furthers current understanding of the early stage development of WMSDs. With future studies’ further research and confirmation, the findings of this study may be able to serve as a first attempt to guide shift scheduling and job design at manufacturing facilities.
The US Department of Labor defines work-related Musculoskeletal Disorders (WMSDs) as musculoskeletal system and connective tissue diseases and disorders, involving overexertion, repetitive motion and vibration that lead to living tissue sprains, strains, tears, as well as pain, swelling, and numbness. MSDs represent one of the leading causes of lost workdays in industry and are associated with major economic costs. Occupational Safety and Health Administration (OSHA) estimated that “work-related MSDs in the United States account for over 600,000 injuries and illnesses and 34 percent of all lost workdays reported to the Bureau of Labor Statistics (BLS, 2016). These disorders now account for one out of every three dollars spent on workers' compensation. It is estimated that employers spend as much as 20 billion dollars a year (U.S.) on direct costs for MSD-related workers' compensation, and up to five times that much for indirect costs, such as those associated with hiring and training replacement workers” (OSHA, 2014).